The ventral photoreceptor of Limulus continues to be a highly productive preparation for detailed electrophysiological studies of phototransduction and light-adaptation: it is large, isolated, and really voltage-clamped. In this proposal, the PI reveals new evidence that: (i) ventral photoreceptors in situ) undergo circadian changes in responsivity to light; and (ii) pharmacological agents, shown to mimic the effects of endogenous efferent neurotransmission in the lateral eye, change function in ventral photoreceptors as well. These findings open up the possibility for uncovering the mechanisms mediating efferent control of photoreceptor gain (change in membrane conductance per photon), shape or duration of quantal bumps, and other cellular mechanisms associated with phototransduction or light-adaptation that are under efferent control. The specific aims of this study are to address the following questions: - What is the specific nature of the changes in photoreceptor gain and bump duration when they are voltage-clamped? - Are these changes the result of specific voltage-sensitive or light-activated channels, and how are these related to the changes in photoreceptor gain and bump duration that occur during efferent neurotransmission? The methods will employ standard and newly developed procedures for the voltage- and patch-clamping of ventral photoreceptors while recording (a) light-intensity functions and (b) i-v (current-voltage) relationships before, during, and after superfusion of specific pharmacological agents (i) that mimic various components of endogenous efferent neurotransmission in lateral eye photoreceptors and (ii) that block specific voltage- dependent conductances. The PI fully expects these studies to provide a deeper understanding of cellular mechanisms involved with phototransduction, light-adaptation, and the biophysics underlying these processes.